My best friends ex-girlfriend of three years wants to get with me (NSFW)

As the title suggests, my best friends ex-girlfriend has vocalized her desire to be physically intimate with me, and I honestly don’t know what to do.

I became friends with both of them around the same time in late high school and continued to support my best friend through their break up, even cutting off his ex for over a year and a half due to their toxicity. I know for a fact that my best friend has never gotten over them, and still is deeply in love with them after three years. Now that the ex-girlfriend has started coming back around, he’s gotten even more paranoid and I’m sure he has on some level an understanding about how she feels.

This is roughly the least amount of detail I can mention but I’ll try to answer any other questions. What should I do about this?